All of the back-to-back testing...
All of the back-to-back testing was run on a Mustang chassis dyno.
For some, the visual appeal of a supercharger is important. There's definitely something to be said for lifting the hood to reveal a positive-displacement supercharger nestled between a pair of Four-Valve cylinder heads. Likewise for a polished centrifugal supercharger packaged with an efficient air-to-water intercooler. To some, this show is every bit as important as the go. Unfortunately, most turbo systems are hidden from view.
One thing the positive-displacement supercharger offers that the turbo would have extreme difficulty duplicating is the instantaneous boost response. As mentioned previously, the Roots-style blower tested on the '03 Cobra motor in our original "Boost Bash" offered an extra 120 lb-ft of torque over the turbo combination at 2,500 rpm. That the turbo eventually caught and passed the supercharged combination (at around 3,700 rpm) means nothing to the blower brigade-to them, torque is king. It can probably be argued that having an extra 120 lb-ft of torque at just 2,500 rpm is nothing more than a recipe for tire spin, but having all that wonderful boost and the attending torque is certainly enticing. The ideal combination would obviously be to have all the low-speed torque offered by the Roots-style (or positive displacement) supercharger combined with the midrange and top-end power of the turbo. The obvious answer is to join the two to produce a compound forced-induction system that offers such an impressive combination. What could be better than having a pair of turbos feeding a Roots-style supercharger?
The turbo kit from HP Performance...
The turbo kit from HP Performance included a set of dedicated tubular headers.
While this compound forced-induction system sounds great on paper, and testing has shown it's capable of producing exceptional power, naturally there are trade-offs associated with such a system.
Despite the recent test run in MM&FF on the combination from Hellion, compound forced induction is far from new. It has been in use almost since the introduction of the internal combustion engine and employed extensively in military and civilian aviation applications. Lancia employed a system on one of its Gruppe B rally cars, and Volkswagen now offers such a system on one of its production cars. Of course, this history doesn't mean we aren't terribly excited about tuners applying it to our wonderful Ford products. This excitement over the compound forced induction has to be (excuse the pun) combined with proper testing to ensure understanding (beyond the usual Internet level) and safe operation. The main reason for this is the tremendous potential for elevated boost pressures associated with a compound system. As we shall see from the test results, compound systems do not simply combine, or add, the boost from the blower to that supplied by the turbo. There's a multiplier effect that quickly elevates the boost pressure and the associated potential for detonation.
Installation of the turbo...
Installation of the turbo kit required a tubular K-member. This provided the necessary room to run the tubing to and from the turbos. All of the testing was run through a 3-inch MagnaFlow dual exhaust equipped with an x pipe system.
To best illustrate the benefits and limitations of compound forced induction, we decided to run a series of dyno tests using an '04 Cobra. The supercharged 4.6L motor was run first with the stock Eaton supercharger, then again with turbos feeding the supercharger (compound induction), and finally using just the turbos. Before getting to the results, it's important to understand a few basic elements of the process. As always, we start with the basic, somewhat simplified, premise that any forced-induction motor is simply a normally aspirated motor with additional atmospheric pressure. That is to say, the supercharged '04 Cobra motor used for this test is nothing more than a normally aspirated Cobra motor with additional pressure supplied. Using this premise, we can estimate the power gains offered by the boost supplied by the supercharger. If we have a 250hp normally aspirated motor and add 11 psi of boost, we can increase the power output by nearly 75 percent, since 11 psi is 75 percent of the normal atmospheric pressure of 14.7 psi. Adding 11 psi to the 250 hp motor should theoretically give us 437 hp. This formula doesn't take into account the effect of changes to timing and air/fuel, changes in the induction system, or losses associated with driving the blower, thus the actual output will likely be lower with a supercharger.
 The tubular headers provided...  The tubular headers provided an unrestricted flow of exhaust energy to the 57mm turbos selected for this test. Note that all of the exhaust components received a thermal barrier coating. Visible in this shot is the header feeding the turbo, the oil drain line back to the pan, and the wastegate. |  The kit from HP Performance...  The kit from HP Performance relied on a pair of Tial wastegates to properly control the boost pressure. The boost curve supplied by the turbos was perfectly flat. |  Boost levels were adjusted...  Boost levels were adjusted by the Holman manual wastegate controller. |